# Lecture 4: Conditional Probability | Statistics 110 | Summary and Q&A

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April 29, 2013
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Harvard University
Lecture 4: Conditional Probability | Statistics 110

## TL;DR

Conditional probability and conditioning involve updating probabilities based on new evidence, with the probability of an event given another event calculated by P(A|B) = P(A∩B) / P(B).

## Key Insights

• 👻 Conditional probability involves updating probabilities based on new evidence, allowing for the consideration of additional information.
• 🌍 Pebble world and frequentist world provide intuitive ways of understanding conditional probability, with the former involving removing irrelevant options and the latter considering long-run frequencies.

## Transcript

SPEAKER: So at the end of last time, we were doing this matching problem, De Montmort. And then I wanted to say a little bit more about that problem, because it was a little rushed at the end of last time. So let's finish talking about that. That was an example of inclusion-exclusion. And then, for the rest of today, I want to introduce independent... Read More

### Q: What is conditional probability?

Conditional probability involves updating probabilities based on new evidence, specifically calculating the probability of event A occurring given that event B has occurred.

### Q: How is conditional probability defined?

Conditional probability is defined by the equation P(A|B) = P(A∩B) / P(B), where P(A|B) represents the probability of event A occurring given event B has occurred.

### Q: What are some intuitive ways to understand conditional probability?

One intuitive way is through the pebble world, where irrelevant pebbles outside of event B are removed and the remaining pebbles represent the restricted universe. Another way is through the frequentist world, which considers long-run frequencies of events A and B occurring.

### Q: What are some important theorems related to conditional probability?

Theorems include the probability of an intersection (P(A∩B) = P(A|B) * P(B)) and Bayes' rule (P(A|B) = P(B|A) * P(A) / P(B)), which relate conditional probabilities to each other.

## Summary

In this video, the speaker introduces the concept of conditional probability and explains how it can be used to update probabilities based on new evidence. The definition of conditional probability is given as the probability of event A occurring given that event B has occurred, which is calculated as the probability of the intersection of A and B divided by the probability of B. Two different intuitions for conditional probability are also provided, including the "pebble world" and "frequentist world" perspectives. The video concludes with the presentation of two theorems related to conditional probability.

### Q: What is the definition of conditional probability?

Conditional probability is defined as the probability of event A occurring given that event B has occurred. It is calculated as the probability of the intersection of A and B divided by the probability of B.

### Q: How can the concept of conditional probability be applied in practice?

Conditional probability allows us to update our probabilities and beliefs based on new evidence. For example, in scientific experiments or investigations, new information may be gained that can influence our understanding of the probabilities involved.

### Q: What are the two intuitions provided for understanding conditional probability?

The first intuition, "pebble world," involves thinking about a sample space and events as pebbles, with the restriction and normalization of probabilities. The second intuition, "frequentist world," considers the long-run frequency of event occurrences when the experiment is repeated many times.

### Q: What are the two theorems related to conditional probability derived in the video?

The first theorem states that the probability of the intersection of events A and B equals the probability of B times the probability of A given B. The second theorem is equivalent but swaps the order to state that the probability of the intersection of events A and B equals the probability of A times the probability of B given A.

### Q: How can conditional probability be useful in updating probabilities based on new evidence?

By calculating conditional probabilities, probabilities can be updated based on new evidence. This allows for a more accurate representation of the likelihood of events occurring, taking into account the information gained from new observations or experiments.

### Q: How does the definition of conditional probability account for the independence of events A and B?

If events A and B are independent, then the probability of A given B would be equal to the probability of A, since the outcome of event B would not provide any new information or affect the likelihood of event A occurring.

### Q: Can you provide an example scenario where conditional probability would be applicable?

One example could be predicting the likelihood of a person having a certain medical condition given their symptoms. By considering the conditional probability of the medical condition given specific symptoms, the probability estimate can be adjusted based on the presence or absence of those symptoms.

### Q: Are there other ways to think about independence besides the definition based on conditional probability?

Yes, there are other equivalent ways to define independence, such as using joint probabilities or conditional probabilities in different combinations. These alternative definitions provide different perspectives on the concept of independence.

### Q: How do the intuitions of "pebble world" and "frequentist world" connect to the definition of conditional probability?

The intuitions of "pebble world" and "frequentist world" offer different perspectives on how to interpret and understand conditional probability. While the definition itself may be more abstract, these intuitions provide concrete examples and ways to think about conditional probability in different scenarios.

### Q: How can conditional probability be used in decision-making or risk assessment?

Conditional probability can be used to assess the likelihood of certain outcomes based on specific conditions or events. This information can then be used to make informed decisions or evaluate risks in various scenarios, such as in insurance, finance, or project management.

## Takeaways

Conditional probability is a fundamental concept in probability theory and statistics. It allows for the updating of probabilities based on new evidence and provides a mathematical framework for adjusting beliefs and uncertainty. Two intuitions, "pebble world" and "frequentist world," offer different perspectives on how to interpret conditional probability. Understanding conditional probability is crucial for many applications, including data analysis, decision-making, risk assessment, and scientific research.

## Summary & Key Takeaways

• Conditional probability allows for the updating of probabilities based on new evidence, with P(A|B) representing the probability of event A occurring given that event B has occurred.

• The definition of conditional probability involves dividing the probability of both events A and B occurring by the probability of event B occurring.

• Pebble world and frequentist world provide different intuitive ways of understanding conditional probability, with pebble world involving removing irrelevant pebbles and frequentist world involving considering long-run frequencies.

• Theorems such as the probability of an intersection and Bayes' rule further explain the calculations and relationships between conditional probabilities.